• Bash History
• Bash Scripts
  • Running as a user
  • debugging bash scripts
  • If statements
  • Misc
• Other pages

Bash History

https://unix.stackexchange.com/a/163406/738908

Mainly controlled by Environment Variables:

• HISTFILESIZE: The maximum number of lines contained in the history file:
  • Default: 2000
  • 0: the history file is truncated to zero size
  • Non-numeric values and numeric values less than zero inhibit truncation.
• HISTSIZE: The number of commands to remember in the command history
  • If the value is 0, commands are not saved in the history list.
  • Numeric values less than zero result in every command being saved on the history list (there is no limit).
  • The shell sets the default value to 100 after reading any startup files.
• HISTIGNORE : This allows you to ignore certain common commands that are rarely of interest. eg: export HISTIGNORE="pwd:df:du"
• HISTCONTROL : This one lets you choose how the history works:
  • ignoredups: save duplicated commands only once
  • ignorespace ignore commands starting with whitespace
  • erasedups: causes all previous lines matching the current line to be removed from the history list before that line is saved
  • ignoreboth: shorthand for ignorespace and ignoredups
• HISTTIMEFORMAT: set the time format of the history file - see man bash

Fine tuning by bash options:

• histappend: the history list is appended to the file named by the value of the HISTFILE variable when the shell exits, rather than overwriting the file. Allows you to combine the histories of different sessions (think different terminals for example).
  • shopt -s histappend OR
  • adding above command to your ~/.bashrc
• The history command has two useful options:
  • history -a : causes the last command to be written to the history file automatically
  • history -r : imports the history file into the current session.

You could, for example, add these two commands to your PROMPT_COMMAND (which is executed each time your shell shows the prompt, so whenever you start a new shell and after each command you run in it): export PROMPT_COMMAND='history -a;history -r;' Combined, they ensure that any new terminal you open will immediately import the history of any other shell sessions. The result is a common history across all terminals/shell sessions.

More history commands which are useful when you have unlimted bash history.

history: displays a numbered list of the commands in the history list history -c: clears the history list !!: runs the last command in the history list !n: runs the nth command stored in history CTRL + R: searches through the history list interactively. To clarify, Bash starts displaying matches as soon as we begin typing what we’re looking for.

From: https://www.baeldung.com/linux/bash-configure-unlimited-history

Bash Scripts

Running as a user

Don’t check via UID, check via username

SPECIALUSERNAME="myusername"
if [ "$(id -un)" != "$SPECIALUSERNAME" ]; then
  echo "Run as special user"
  exit
fi

If you really really want to check via UID

if [ "$EUID" -ne 1 ]; then
  echo "Please run as root, maybe via sudo"
  exit
fi

debugging bash scripts

Based off: https://unix.stackexchange.com/questions/155551/how-to-debug-a-bash-script

bash -x ./script.sh OR
add set -x in your script to see debug output. You can also use set to only debug sections:

set -x
echo "this is the part of the script I want to debug."
set +x

In Bash 4.1 or later you can put the debug output into a separate file by adding the below into your script:

exec 5> debug_output.txt
BASH_XTRACEFD="5"

If you want line numbers add this:

PS4='$LINENO: '

See https://unix.stackexchange.com/questions/155551/how-to-debug-a-bash-script for examples of using the logger command in your script.

If you really want better logging in your bash script, you could look into log4bash

TLDP Debugging Guide: https://tldp.org/LDP/Bash-Beginners-Guide/html/sect_02_03.html

If statements

Form: https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html

-a file
True if file exists.

-b file
True if file exists and is a block special file.

-c file
True if file exists and is a character special file.

-d file
True if file exists and is a directory.

-e file
True if file exists.

-f file
True if file exists and is a regular file.

-g file
True if file exists and its set-group-id bit is set.

-h file
True if file exists and is a symbolic link.

-k file
True if file exists and its "sticky" bit is set.

-p file
True if file exists and is a named pipe (FIFO).

-r file
True if file exists and is readable.

-s file
True if file exists and has a size greater than zero.

-t fd
True if file descriptor fd is open and refers to a terminal.

-u file
True if file exists and its set-user-id bit is set.

-w file
True if file exists and is writable.

-x file
True if file exists and is executable.

-G file
True if file exists and is owned by the effective group id.

-L file
True if file exists and is a symbolic link.

-N file
True if file exists and has been modified since it was last read.

-O file
True if file exists and is owned by the effective user id.

-S file
True if file exists and is a socket.

file1 -ef file2
True if file1 and file2 refer to the same device and inode numbers.

file1 -nt file2
True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.

file1 -ot file2
True if file1 is older than file2, or if file2 exists and file1 does not.

-o optname
True if the shell option optname is enabled. The list of options appears in the description of the -o option to the set builtin (see The Set Builtin).

-v varname
True if the shell variable varname is set (has been assigned a value).

-R varname
True if the shell variable varname is set and is a name reference.

-z string
True if the length of string is zero.

-n string
string
True if the length of string is non-zero.

string1 == string2
string1 = string2
True if the strings are equal. When used with the [[ command, this performs pattern matching as described above (see Conditional Constructs).

‘=’ should be used with the test command for POSIX conformance.

string1 != string2
True if the strings are not equal.

string1 < string2
True if string1 sorts before string2 lexicographically.

string1 > string2
True if string1 sorts after string2 lexicographically.

arg1 OP arg2
OP is one of ‘-eq’, ‘-ne’, ‘-lt’, ‘-le’, ‘-gt’, or ‘-ge’. These arithmetic binary operators return true if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively. Arg1 and arg2 may be positive or negative integers. When used with the [[ command, Arg1 and Arg2 are evaluated as arithmetic expressions (see Shell Arithmetic).

Misc

• It’s good practice to not have the tailing slash on directories. Some tools distinguish between directories with a trailing slash and without
• Use exit codes. It stops error propagating. See man errno for standard numbers
• Use stderr for error messages, eg >&2 echo "ERROR: ..."

Other pages

See Aliases in Linux Commands
See Builtin Bash Commands in Linux Commands
See Bash Reference Manual in HTML Book form or Other forms

Linux Bash Shell Scripting Tutorial Wiki